1. Field of the Invention
The present invention relates to an improvement of an arc processing apparatus of a system for use in an arc process such as an arc welding process, an arc cutting process, an arc heating process, an arc melting process or the like, and in particular, to an arc processing apparatus of a system in which arc is started by once short-circuiting an electrode with a workpiece to be processed and thereafter separating the electrode from the workpiece in arc processing. In more particular, the present invention relates to an arc processing apparatus comprising a driving means for controlling an output transistor so that an output voltage from the output control transistor becomes a predetermined no-load voltage.
2. Description Of the Related Art
FIG. 6 is a circuit diagram of an exemplified prior art arc processing apparatus.
Referring to FIG. 6, the prior art arc processing apparatus comprises a welding power supply 10, and the followings are connected to the welding power supply 10:
(a) primary power input lines 1 through 3; PA1 (b) an electrode 4 through an electrode cable 6; PA1 (c) a workpiece 5 to be processed through a workpiece cable 7; PA1 (d) ground through a torch switch 8, with an activation ground signal being inputted to a controller CL1 when the torch switch 8 is turned on; and PA1 (e) a remote output adjuster 9, with an output control signal having a control voltage determined by the remote output adjuster 9 being inputted to the controller CL1. PA1 a main power source for supplying an electric power for arc process through said output terminals to said load; PA1 an auxiliary power source for supplying another electric power through said output terminals to said load, said auxiliary power source having a no-load voltage higher than that of said main power source and an electric power capacity smaller than that of said main power source; PA1 first output switching means for turning on or off supply of the electric power from said main power source; PA1 second output switching means for turning on or off supply of the electric power from said auxiliary power source; PA1 arc detecting means for detecting generation of an arc by detecting whether or not a voltage between said output terminals is lower than a predetermined threshold voltage, and outputting an arc detection signal; PA1 control means for turning on said second output switching means so as to short-circuit said electrode with said workpiece and to perform an arc activation, and thereafter, in response to the arc detection signal, for turning on said first output switching means; PA1 output control transistor electrically connected in series between said auxiliary power source and said output terminals; and PA1 driving means for controlling said output control transistor so that an output voltage from said output terminals becomes a predetermined no-load voltage in a range of an output current flowing in said output terminals from zero to a predetermined threshold current. PA1 a DC reactor electrically connected in series between said output control transistor and said output terminals. PA1 a further DC reactor electrically connected in series between said main power source and said output terminals. PA1 The above-mentioned arc processing apparatus further comprises: PA1 a first DC reactor electrically connected in series between said main power source and said output terminals; and PA1 a second DC reactor electrically connected in series between said output control transistor and said output terminals; PA1 wherein windings of said first DC reactor and windings of said second DC reactor are wound around an iron core so that magnetic fluxes in the same direction are generated when currents flow in both of the windings of said first DC reactor and the windings of said second DC reactor respectively from said main power source and said auxiliary power source in an identical direction. PA1 a current limiting resistor for providing a drooping characteristic in an output voltage to an output current characteristic of said auxiliary power source, said current limiting resistor being electrically connected in series between said auxiliary power source and said output control transistor. PA1 feedback control means for providing a constant-current characteristic in a range of the output current from zero to the predetermined threshold current, and providing a drooping characteristic in another range of the output current from the predetermined threshold current to a predetermined short-circuit current, in an output voltage to an output current characteristic of said auxiliary power source.
The welding power supply 10 includes a primary rectifier DR1, a smoothing electrolytic capacitor C1, an inverter TR1, an inverter transformer T1, a secondary rectifier DR2, a DC reactor LD1, an output current detector CT1, an auxiliary power rectifier DR3, a smoothing electrolytic capacitor C2, a switch S1, a current limiting resistor R1, an auxiliary power current detector CT2, a thyristor SCR1, a voltage detector DT1, the controller CL1, an electrode-use output terminal TM1, and a workpiece-use output terminal TM2.
In FIG. 6, the primary rectifier DR1, the smoothing electrolytic capacitor C1, the inverter TR1, the inverter transformer T1, the secondary rectifier DR2, the DC reactor LD1, the output current detector CT1 and the controller CL1 constitute a main power source MPS for supplying a DC electric power for an arc process. On the other hand, the auxiliary power rectifier DR3, the smoothing electrolytic capacitor C2, the current limiting resistor R1 and the auxiliary power current detector CT2 constitute an auxiliary power source APS for supplying a DC electric power for an arc activation or start for the arc process.
The switch S1 is a switching means for turning on and off the auxiliary power source APS, while the thyristor SCR1 is another switching means for turning on and off the main power source MPS. Further, an output characteristic of the main power source MPS becomes a constant-current characteristic achieved under a constant-current control through a feedback control by the output current detector CT1, the controller CL1 and the inverter TR1, while an output characteristic of the auxiliary power source APS becomes a drooping characteristic achieved by the current limiting resistor R1.
FIG. 7 is a graph showing an output characteristic of the prior art arc processing apparatus shown in FIG. 6.
As shown in FIG. 7, a no-load voltage Vst1 of the auxiliary power source APS is set to be higher than a no-load load voltage Vst2 of the main power source MPS, and a short-circuit current Is2 of the main power source MPS is set to be greater than a short-circuit current Ist1 of the auxiliary power source APS.
In the arc processing apparatus shown in FIG. 6, when the torch switch 8 is pressed so as to be turned on, an activation ground signal is inputted to the controller CL1. In response to the activation ground signal, the controller CL1 drives a gas supply circuit (not shown). The gas supply circuit discharges a shield gas a location between the electrode 4 and the workpiece 5 which are opened with respect to each other. Taking into consideration a time necessary for the shield gas to reach the electrode 4, when the shield gas reaches the electrode 4, the controller CL1 activates the inverter TR1 and turns on the switch S1 with the thyristor SCR1 put in the OFF-state thereof. Upon this operation, the no-load voltage Vst2 of the main power source MPS is not applied to a load of air between the electrode 4 and the workpiece 5 since the thyristor SCR1 is in the OFF-state thereof, and the no-load voltage Vst1 of the auxiliary power source APS is applied to the load therebetween. When the electrode 4 and the workpiece 5 are short-circuited with each other in this state, the short-circuit current Is1 shown in FIG. 7 flows in the load therebetween.
Subsequently, when the electrode 4 is separated from the workpiece 5 and there is generated a relatively small arc smaller than a welding arc generated upon the arc process, the voltage between the output terminals TM1 and TM2 decreases. The voltage detector DT1 detects whether or not an arc is generated by detecting whether or not the voltage between the output terminals TM1 and TM2 becomes lower than a predetermined detection voltage, and outputs an arc detection signal upon generation of the arc. The controller CL1 receives the arc detection signal outputted from the voltage detector DT1, and then, the controller CL1 decides that an arc has been generated. Then, the controller CL1 makes the thyristor SCR1 conductive, and subsequently turns off the switch S1. Consequently, an electric power starts to be supplied from the main power source MPS to the relatively small arc generated in the load containing the electrode 4 and the workpiece 5, and then, the relatively small arc grows into a welding arc. If the arc vanishes when the electrode 4 is separated from the workpiece 5, the controller CL1 does not make the thyristor SCR1 conductive, so that no electric power is supplied from the main power source MPS.
In the prior art apparatus shown in FIG. 6, the relatively small arc generated when the electrode 4 is separated from the workpiece 5 is a relatively small current, and therefore, the generated arc becomes unstable. When the no-load voltage Vst2 of the main power source MPS is relatively low, the arc tends to vanish. However, if the no-load voltage Vst2 is increased, an electric power capacity of the arc processing apparatus increases depending on (no-load voltage).times.(rated output current), and this leads to an expensive manufacturing cost. Furthermore, even in the case where the relatively small arc generated when the electrode 4 is separated from the workpiece 5 does not vanish, no stable small arc can be formed when the short-circuit current Is1 of the auxiliary power source APS is relatively small, and therefore, the relatively small arc sometimes cannot be switched or shifted into a welding arc.
FIG. 8 shows a graph for explaining the output. characteristic of the prior art are processing apparatus.
In order to make the relatively small arc smoothly be switched or shifted into a welding arc, the current in the stage is required to be increased. However, when the no-load voltage Vst1 of the auxiliary power source APS is not changed, the value of the current limiting resistor R1 is required to be reduced so that the short-circuit current of the auxiliary power source APS becomes a current Is3 greater than Is1. However, when the resistance value of the current limiting resistor R1 is reduced so as to increase the short-circuit current, this results in a gradual inclination of the drooping characteristic, and consequently the stability of the relatively small arc is reduced. On the contrary, in order to increase the short-circuit current without changing the inclination of the drooping characteristic, the no-load voltage of the auxiliary power source APS is required to be increased to a relatively large voltage, for example, a voltage Vst3 as shown in FIG. 8. In such a case, there is such a problem that it is necessary to provide the auxiliary power source APS having a relatively large electric power capacity.